Curing is a term used in polymer chemistry to define the toughening or hardening of polymeric substance, through cross-linking of polymer chains or otherwise. Curing is generally effected by addition of chemical additives, ultraviolet radiation, electron-beam radiation, heat or other activators. The curing of rubber compounds, whether natural rubber or synthetic rubber, is known as “vulcanization”. Hereinafter, the term “rubber” will be used to describe both natural rubber and synthetic rubber compounds and mixtures.
Uncured natural rubber and synthetic rubber compounds such as polychloroprene tend to be sticky and deformable at relatively warm temperatures, while being brittle and rigid at relatively cold temperatures. In the natural state, rubber may be relatively inelastic and may undergo a high degree of inelastic deformation due to its long polymer chains which may move against each other freely. Vulcanization prevents or reduces the relatively free movement of these polymer chains through cross-linking, resulting in more elastic deformation when stress is applied to the cured rubber, and a return to the original shape when the stress is removed. Vulcanized rubber products tend to be less sticky or tacky than equivalent non-vulcanized products, and generally have superior mechanical properties such as durability, hardness, abrasion resistance and the like. A vast number of products containing vulcanized rubber are available including rubber tyres, soles for shoes and boots, hose pipes, rubber belts, sporting equipment, rubber flooring etc. The degree of vulcanization of rubber can be tailored to suit whichever application the resultant rubber is to be applied to, and different physical and mechanical properties can be imparted to rubber through vulcanization through use of different curing and vulcanization additives and activators.
Vulcanization is generally irreversible. The cross-linking of the rubber polymer chains is usually done with sulphur, urethane cross-linkers, metallic oxides, acetoxysilane or peroxide-based systems.
The main polymers subjected to vulcanization are polyisoprene (natural rubber), polychloroprene (CR), styrene-butadiene rubber (SBR) and any other rubber containing a diene in the polymer chain. The curing regime is adjusted for the substrate and the application of the final rubber product. The reactive sites in most rubber compounds are allylic hydrogen atoms and these C—H bonds are adjacent to carbon-carbon double bonds. In sulphur-based vulcanization some of the C—H bonds are replaced by chains of sulphur atoms that link with a reactive site of another polymer chain. These bridges contain between one and eight atoms. The number of sulphur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short crosslinks give the rubber better heat resistance while longer crosslinks give the rubber good dynamic properties but with less heat resistance.
Sulphur itself is relatively slow as a curing agent, even using industry standard high temperature and pressure curing processes, and even using large quantities of sulphur does not generally produce vulcanized rubber having adequate physical properties. In order to speed up vulcanization or curing processes, whether sulphur-based vulcanization, or using any other curing additive or activator, a curing regime is usually effected which may include multiple types of additive and/or activator, and which includes retarding agents that inhibit vulcanization for a predetermined time or until a specific temperature is reached, antidegradants to prevent degradation of the vulcanized product by heat, oxygen, UV and ozone and “accelerators”.
Accelerator compounds and compositions are known for use in accelerating polymeric curing and rubber vulcanization. Many different accelerators are known, and their use will depend on the specific rubber compound(s) to be cured and the intended physical properties of the cured rubber. Activator compounds may also be used to activate accelerator compounds or curing per se. Finally in some processes, curing occurs very quickly and it can be desirable to utilise retardants or retarding agents to retard the onset of cure, cure rate or extent of curing or vulcanisation.
The vulcanization of neoprene or polychloroprene rubber (CR) is generally carried out using metal oxides such as MgO and ZnO in the presence of an accelerator. In addition, because of various processing factors, including “scorch” (the premature cross-linking of rubbers due to the influence of heat), the choice of accelerator is governed by different conditions to other diene rubbers. The primary accelerator in use today for accelerating the vulcanization of CR is ethylene thiourea (ETU) which, although being a proven accelerator for polychloroprene, has been classified as reprotoxic and environmentally damaging.
It would therefore be advantageous to provide an accelerator composition which can be used to accelerate the curing or vulcanization of rubber, whether natural or synthetic, which mitigates at least one of the problems of the prior art.
It would also be advantageous to provide an accelerator composition which can replace ethylene thiourea in acceleration of polychloroprene curing and curing of other natural and synthetic rubber compounds and mixtures.